This type of a pressure reducing valve (regulator) typically includes an open/close valve arranged between a primary port into which high-pressure gas flows and a secondary port from which the gas is discharged. The open/close valve opens and closes to reduce the pressure (primary pressure) of the high-pressure gas flowing through the primary port to a secondary pressure and discharges the gas with the reduced pressure.
In the prior art, such pressure reducing valves include a piston-type pressure reducing valve. The piston-type pressure reducing valve includes a cylinder arranged downstream from an open/close valve and a piston arranged within the cylinder to a slidable manner and defining a pressure reducing chamber and a pressure regulating chamber within the cylinder. The piston moves in accordance with the pressure difference between the pressure regulating chamber side and the pressure reducing chamber side to open or close the open/close valve. Such a pressure reducing valve is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2004-192462.
The piston-type pressure reducing valve normally includes a seal arranged on the outer circumferential surface of the piston. The seal ensures sealing (hermetic sealing) between the pressure reducing chamber and the pressure regulating chamber. In many cases, an O-ring is used as the seal.
However, for a hydrogen tank used, for example, in a fuel cell vehicle, there is a trend for increasing the pressure (e.g. 70 MPa) to increase storage capacity. A pressure reducing valve for such a hydrogen tank would be subjected to an extremely high gas pressure that is applied to the pressure receiving surface of a piston at the side of the pressure reducing chamber and to the seal. Thus, when the pressure reducing valve uses an O-ring as its seal to ensure sufficient sealing in the same manner as a normal pressure reducing valve, the O-ring must be set at an extremely high compression rate. In this case, the friction between the O-ring and inner circumferential surface of the cylinder increases. This interferes with movement of the piston.
Accordingly, in the prior art, the pressure reducing valve for high-pressure gas conventionally includes an annular seal, or a “lip seal,” which has a slide piece (seal lip) that slides in contact with the inner circumferential surface of the cylinder due to the pressuring force of an elastic member. The lip seal is arranged on the outer circumferential surface of the piston to ensure sealing.
For example, the pressure reducing valve 71 shown in FIG. 5 uses a lip seal 72 including a resin ring member 73, which has a U-shaped cross-section, and an elastic member 75. The elastic member 75, which is formed by an annular spring member, has a U-shaped cross-section and is arranged in a recess 74 of the ring member 73. The lip seal 72, which is fitted in an accommodating groove 77 formed on an outer circumferential surface of a piston 76, is fixed to the piston 76 by a fastening force of a fastening nut 81 that is mated with a threaded portion 80 formed on a head portion 78 of the piston 76. The elastic force of the elastic member 75 presses a seal lip 82 of the ring member 73 toward a cylinder inner circumferential surface 83 until coming in contact with the cylinder inner circumferential surface 83. This achieves the sealing between a pressure reducing chamber and a pressure regulating chamber.
However, an engagement portion 84 defined between the threaded portion 80 and the fastening nut 81 needs to have play in the radial direction. In other words, a margin must be provided for engagement of the threads of the threaded portion 80 and the fastening nut 81. Even if the piston 76, the threaded portion 80, and the fastening nut 81 are formed in a manner that they are arranged coaxially, the play results in a backlash. Thus, the piston 76 and the fastening nut 81 may not necessarily be arranged coaxially. In the prior art, taking into consideration such backlash, the outer diameter of the fastening nut 81 is smaller than the outer diameter of the piston 76.
However, to regulate the pressure of ultrahigh-pressure gas as described above, the pressing force for pressing the seal lip 82 against the cylinder inner circumferential surface is also se to be extremely high in correspondence with the gas pressure. Due to the frictional force, the lip seal 72 easily deforms in the axial direction when the piston 76 moves. Repetitive reciprocation of the piston 76 may result in the seal lip 82 becoming caught in the gap formed between the fastening rut 81 and the piston 76. This would interfere with smooth movement of the piston 76 and consequently lower the regulation accuracy of the gas pressure.